NUCLEAR FUEL GRINDING REDUCTION

REDUCTION PAR BROYAGE DE COMBUSTIBLES NUCLEAIRES

English Abstract

ABSTRACT OF THE DISCLOSURE
The reduction of powered substance by mill grinding
in a slurry may utilize passivated iron bodies such as balls
or rods. In the case of nuclear fuels containing primarily
uranium material the neutral to acidic reaction of the slurry
may be countered by the use of ammonium hydroxide, by means
of which the viscosity of the slurry is significantly reduced
so that the loading of the mill can be increased by a
significant multiplier, while the relative moisture content
of the resulting reduced slurry also is correspondingly
reduced, to facilitate the drying thereof.

Claims

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The embodiments of the invention in which an exclu-
sive property or privilege is claimed are defined as follows:
1. The method of producing a powder material of
reduced particle size from a slurry using attrition by
abrading within a mill, comprising:
the improvement of carrying out said abrading step
in the presence of ammonium hydroxide to achieve a desired
particle size distribution while reducing the viscosity of
the slurry to permit enhanced utilization of the process.
2. The method as claimed in claim 1 including the
step of reducing the relative weight of water present in
said slurry.
3. The method as claimed in claim 2 including
the step of heating said abraded slurry to decompose said
ammonium hydroxide.
4. The method as claimed in claim 1, claim 2 or
claim 3 wherein said powder material comprises uranium oxide.
5. The method as claimed in claim 1, claim 2 or
claim 3 wherein said slurry material comprises a mixture of
UO2 and other oxides of uranium and/or plutonium oxide or
mixtures thereof where oxides of uranium are the major
constituent.
6. In a process for abrading an aqueous slurry
containing uranium oxide the improvement comprising introducing
ammonium hydroxide whereby the viscosity of the slurry is reduced.
7. The process as claimed in claim 6 wherein said
ammonium hydroxide is added in liquid form to said slurry.
8. The process as claimed in claim 6 wherein
said ammonium hydroxide is generated in-situ.
9. The process as claimed in claim 6, claim 7 or
claim 8 wherein the pH value of said slurry having ammonium
hydroxide combined therewith is in the approximate range
pH 9 to 12.

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10. The process as claimed in claim 6, claim 7
or claim 8 including the step of heating said abraded
slurry in order to disperse said ammonium hydroxide from the
said components of the slurry.

Description

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This invention is directed to a powder production
system, and in particular to the reduction in particle size
of a normally acid slurry of uranium oxide, using a grinding
mill arrangement wherein mill loading is significantly
increased, and energy requirement for milling and drying are
significantly reduced.
In the preparation of fabricated sintered bodies from
powdered starting materials the control of particle size
plays a significant role in the quality of the final product,
and the cost of preparing the powder in the desired particle
size range contributes significantly to the final cost of the
fabricated body. The cost of powder production includes a
cost factor for reducing the particle size of the powder and
a cost factor for drying the powder.
In the case of milling reduction of nuclear fuel
material it has been found that the normal load or batch of
powder which the reduction system can handle can be
dramatically increased in the order of about 200 percent,
i.e. by a factor of approximately 3, by reducing the viscosit~
of the slurry while maintaining the gross liquid content
thereof substantially unchanged.
Reduction in slurry viscosity is effected by the
use of ammonium hydroxide. In one instance, in an attrition
mill utilizing balls of low carbon steel as the grinding
medium, and incorporating a slurry circulation pump to
recirculate the slurry through the mill, with an initial
batch load of three hundred gallons of water and five thousand
pounds of uranium oxide fuel, the viscosity of the slurry
within the mill was such that the capacity of the pump was
reached, and the concentration of uranium oxide thus limited.
By the addition of six gallons of ammonium hydroxide, a low
cost, commercially available chemical the viscosity of the slurry
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reduced dramatically, so that a further ten thousand pounds
to twelve thousand pounds of uranium oxide could be added to
the system, and still handled by the pump without overloading.
Owing to the fact that the quantity of liquid is
virtually unchanged, the solid to liquid weight ratio of the
slurry increases from approximately 166% to upwards of
500%. Thus, in drying the slurry the weight of water per
pound of slurry diminishes from 0.375 pounds per pound to
the range 0.167 - 0.15 pounds per pound.
The reduction in water per pound of slurry
correspondingly diminishes slurry drying costs, generally in
the same order of saving i.e. at least 50% saving.
A further effect of grind reducing the oxide as
a more heavily concentrated slurry is that the efficiency of
grinding is enhanced using the greater slurry density, to
show a saving in grinding time in the order of 10% or better,
per pound of slurry, which corresponds with an energy saving
of 10% in the grinding section and 50% in the drier section,
with corresponding reduction in plant size or utilization,
of 10% in the grinder and 50% in the dryer.
A further and most significant benefit is the
reduction of water consumption required to process the oxide
fuel, the water consumption improvement also being by a factor
of three. Thus 3000 pounds of water services 15,000 to 17,000
pounds of uranium oxide, as compared with the former require-
ment of 3000 pounds of water servicing 5000 pounds of uranium
oxide. While in the past the cost of water has ~een reglected,
this is no longer the case in our present ecology.
The avoidance of iron contamination in the subject
process does not form a part of the present invention, being
C~na.~l~ n~6~r ~5~,~3 ~
dealt with in a copending~applicationAin the name of the
present inventors. However, the use of ammonium hydroxide in
,

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1O~5L~81
the manner taught herein has been found to contribute
significantly to the passivation of iron in component parts
of the mill system.
The weight of ammonium hydroxide used comprises
1 to 4 weight % of the total weight of the charge.
In the case of grinding nuclear fuel the use of
ammonium hydroxide has the particular advantage that the
subsequent processing step, that of heating the reduced slurry,
has the particular advantage that the ammonium hydroxide is
totally decomposed and driven off from the fuel.
The slurry concentration achieved comprises 65
to 80 weight percent of UO2 in the UO2 plus H2O mixture.
In the case of other slurries it is postulated that
the presence of the ammonium ion may be the root cause of the
viscosity reduction, in which case it may prove possible to
achieve the benefits of the invention using sources of the
ammonium ion other than ammonium hydroxide in the slurry. The
pH value for which significant benefit of viscosity reduction
is obtained appears to lie in the range pH 9 to 12, in the
case of ammonium hydroxide.
Ammonium hydroxide can be generated in-situ, at
least in the case of the aqueous slurry of nuclear fuel,
by bubbling ammonium gas into the slurry.